The present invention relates to a receiving system as a component of a wireless communication system and a semiconductor integrated circuit device for processing a wireless communication signal, having therein a receiving circuit and a transmitting circuit. More particularly, the invention relates to a receiving system having a quadrature mixer for performing frequency conversion on a signal by using two local signals having the same frequency but whose phases are different from each other by 90 degrees and to a semiconductor integrated circuit device for processing a wireless communication signal.
Hitherto, a wireless communication signal processing circuit is constructed by using separate parts for each of function blocks (an amplifier for amplifying a signal, a mixer for converting the frequency of a signal, a filter for passing only a desired band in a signal, and the like). Because of improvement in the semiconductor technique in recent years, a plurality of function blocks constructing the wireless communication signal processing circuit can be provided in one semiconductor chip. The wireless communication signal processing circuit built in one or a plurality of semiconductor chips converts a radio frequency signal received from an antenna to a lower frequency signal of high qualities (low noise, high linearity, suppression of signals in bands other than the desired band, and the like). There is also an IC for processing a wireless communication signal, in which a transmitting circuit for converting a signal supplied from a baseband part to a signal of a higher frequency band is provided together with a receiving circuit in a single semiconductor chip.
To realize the wireless communication signal processing circuit device at low cost, a larger number of function blocks constructing the wireless communication signal processing circuit have to be provided in a single semiconductor chip. One of problems against the object is that a filter circuit for suppressing signals in bands other than the desired band has to be provided in the semiconductor chip. Generally, as the filter circuit, an SAW (Surface Acoustic Wave) filter, a dielectric filter, and the like is used. By such a filter, signals existing in bands other than the desired band are suppressed. However, the SAW filter and the dielectric filter cannot be provided in a semiconductor chip.
The wireless communication signal processing circuit device in an individual part generally has a configuration called a superheterodyne configuration and needs an SAW filter or dielectric filter. However, such a filter cannot be provided in the semiconductor chip. Consequently, when a wireless communication signal processing circuit device manufactured of semiconductor is constructed as a superheterodyne device, an SAW filter or a dielectric filter has to be provided on the outside of the semiconductor chip. It increases the number of parts and the mounting area.
A wireless communication signal processing circuit device has also been proposed, which does not require an SAW filter or a dielectric filter by using the advantage of a semiconductor circuit such that although absolute values of constants of parts of semiconductor chips vary, the absolute values of constants of parts of a single semiconductor chip match each other with high precision. This method includes a zero-IF method and a low IF method. None of the methods requires an external SAW filter or a dielectric filter and suppresses signals existing in bands other than the desired band by using a filter which can be provided in the semiconductor chip. It may be necessary to externally provide a part of filters depending on a wireless communication method or a requirement from the viewpoint of the system.
The radical principles of the zero-IF method, low IF method, and the like are described in, for example, Non-patent Reference 1. The zero-IF method and the low IF method have a common operational characteristic such that a signal is decomposed to two components of an I component and a Q component, and the two components are processed. By inputting two local oscillation signals having the same frequency and having phases different from each other by 90 degrees and a signal to be decomposed to the I component and the Q component to a quadrature mixer, the signals are decomposed to the I and Q components.
To explain the operation, FIG. 6 shows a basic configuration of a zero-IF wireless communication signal receiving system. Shown in FIG. 6 are an antenna input terminal 1, a band pass filter (hereinbelow, abbreviated as “BPF”) 2, a low noise amplifier (hereinbelow, referred to as “LNA”) 3, quadrature mixers 4I and 4Q, low pass filters (hereinbelow, abbreviated as “LPFs”) 5I and 5Q, amplifiers 6I and 6Q, a 90-degree phase-shifting circuit 7, a local oscillator 8, and output terminals 9I1, 9I2, 9Q1 and 9Q2. Each of the blocks 2, 3, 4I, 4Q, 5I, 5Q, 6I, and 6Q has a differential configuration and the reference numerals with subscripts o and oB 2o, 2oB, 3o, 3oB, 4Io, 4IoB, 4Qo, 4QoB, 5Io, 5IoB, 5Qo, 5QoB, 6Io, 6IoB, 6Qo, and 6QoB denote output signals from terminals. Each of the terminals outputs output signals whose phases are different from each other by 180 degrees.
A wireless communication signal input from the antenna input terminal 1 is supplied to the BPF 2 where signals in bands other than a desired band are suppressed, and the resultant signal is input as a balanced signal to the LNA 3. The LNA 3 amplifies an output signal of the BPF 2 so as not to degrade the signal-to-noise ratio (hereinbelow, abbreviated as “SNR”) as much as possible. An output signal of the LNA 3 is equally divided into two signals and the signals are input to the quadrature mixers 4I and 4Q. An output signal of the local oscillator 8 is subjected to 90-degree phase shifting by the 90-degree phase shifting circuit 7 and the resultant is input as a local oscillation signal to the quadrature mixer 4I. To the quadrature mixer 4Q, an output signal of the local oscillator 8 is input as a local oscillation signal without being subjected to phase shifting. At this time, the frequency of an output signal of the local oscillator 8 coincides with the center frequency of a signal in a desired channel of a wireless communication signal input from the antenna input terminal 1.
Therefore, an output signal of the quadrature mixer 4I becomes an I component of a signal in a desired band of the wireless communication signal input from the antenna input terminal 1, and an output signal of the quadrature mixer 4Q becomes a Q component of a signal in a desired band of the wireless communication signal input from the antenna input terminal 1. Output signals of the quadrature mixers 4I and 4Q are called normal-band signals in the zero-IF method.
The LPFs 5I and 5Q function as channel selecting filters and suppress bands other than the band of the desired channel signal. Output signals of the LPFs 5I and 5Q are amplified to a desired level by the amplifiers 6I and 6Q, and the amplified signals are output from the output terminals 9I1, 9I2, 9Q1 and 9Q2.
The output signal o from each of the blocks having the differential configuration includes an opposite phase component having a phase different from that of the output signal oB by 180 degrees and also includes the in-phase signal components having the same phase. Therefore, when the in-phase signal components of the output signals o and oB are not the same, an offset occurs. When the offset is large and exceeds a dynamic range of a block at a post stage, the opposite-phase signal components of the output signals o and oB as desired signal components cannot be processed.
To reduce the offset, the in-phase signal components in the output signals o and oB are averaged. FIG. 7 shows an example of the configuration of the zero-IF wireless communication signal receiving circuit to explain the above. In FIG. 7, 10I and 10Q denote in-phase signal component averaging circuits. In FIG. 7, the same reference numerals as those of FIG. 6 are given to components performing operations similar to those of FIG. 6 and their description will not be repeated here. The in-phase signal component averaging circuit 10I averages the in-phase signal components in the output signals 4Io and 4IoB of the quadrature mixer 4I to reduce the offset of the quadrature mixer 4I. The in-phase signal component averaging circuit 10Q averages the in-phase signal components in the output signals 4Qo and 4QoB of the quadrature mixer 4Q and reduces the offset of the quadrature mixer 4Q.
The in-phase signal component averaging circuits 10I and 10Q are constructed, for example, as shown in FIG. 8. In FIG. 8, the same reference numerals as those of FIG. 6 are given to components performing operations similar to those of FIG. 6 and their description will not be repeated here. 11I and 11Q denote passive elements. The passive elements 11I and 11Q have an equal impedance. In the case where the in-phase signal component of the output signal 4IoB of the quadrature mixer 4I is larger than that of the output signal 4Io, correction current passing through the passive element 11I flows from 4Io to 4IoB, and an offset of the quadrature mixer 4I is reduced. In the case where the in-phase signal component of the output signal 4QoB is larger than that of the output signal 4Qo of the quadrature mixer 4Q, correction current passing through the passive element 11Q flows from 4Qo to 4QoB, and the offset of the quadrature mixer 4Q is reduced. The passive elements 11I and 11Q suppress both the in-phase components and the opposite phase components of signal components out of the desired channel bands of the output signals 4Io, 4IoB, 4Qo, and 4QoB.
In the receiving system of FIG. 8, an offset is reduced by the passive elements 11I and 11Q. However, due to variations in characteristics of elements constructing the circuit, even when the passive elements 11I and 11Q are used, a large offset which cannot satisfy the specifications may occur. When the offset of the quadrature mixer 4I satisfies the specifications but the offset of the quadrature mixer 4Q does not satisfy the specifications or vice versa, the wireless communication signal receiving circuit is defective and the manufacturing yield deteriorates. When the impedances of the passive elements 11I and 11Q are reduced, an offset also decreases. However, the opposite phase component of a desires signal in a desired channel band also attenuates and the SNR deteriorates. Although elimination of an offset by a digital process is proposed in Patent Reference 1, it cannot be realized at low cost for reasons like the circuit cannot be provided in a semiconductor chip.
Although the problems in the receiving system have been described above, in a manner similar to the receiving system, a transmitting system is also provided with a frequency converter for inputting a signal to be transmitted and a local oscillator to a quadrature mixer, converting a signal to a signal of a higher frequency, and outputting the resultant signal. Consequently, also in the quadrature mixer on the transmission side, there is a problem that a DC offset occurs between differential output signals due to variations in the characteristics of elements of the circuit.
Further, the transmitting system has an amplifier for amplifying a signal to be transmitted on the ante stage of the quadrature mixer. When a DC offset occurs in output signals of the amplifier due to variations in the characteristics of the elements of the amplifier, a carrier leak occurs in which in addition to a desired frequency, a frequency component which is the same as that of a carrier wave appears in an output of the quadrature mixer, and the yield deteriorates.
When the wireless communication system using the transmitting system is a portable telephone of GSM (Global System for Mobile Communication) in which transmission and reception are performed separately, it is possible to provide an amplifier with a circuit for compensating for a DC offset in output signals, measure a DC offset of the amplifier during reception, and compensate for the offset. In the case of a portable telephone of the CDMA (Code Division Multiple Access) for performing transmission and reception concurrently, there is no time allowance for performing measurement and compensation for a DC offset. Consequently, there is a problem such that a DC offset occurs due to characteristic variations in elements constructing the circuit.    [Patent Reference 1]
Japanese Unexamined Patent Publication No. 11(1999)-55342    [Non-patent Reference 1]
“Direct Conversion Receivers in Wide-Band Systems” by Aarno Parssinen, Kluwer Academic Publishers